The invention relates to aqueous two-component polyurethane systems comprising vinyl polymer polyol dispersions modified with reactive diluents, to a process for preparing them and to use as coating materials.
Aqueous two-component (2K) polyurethane (PUR) systems have been described in the prior art, for example in EP-A 0 358 979, EP-A 0 496 210 and EP-A 0 557 844. Both so-called secondary dispersions and so-called primary dispersions are employed as polyol components, and are combined with appropriate polyisocyanates.
“Secondary dispersions” are those aqueous dispersions which are first polymerized in a homogeneous organic medium and then redispersed in an aqueous medium with neutralization, generally without the addition of external emulsifiers.
“Primary dispersions” are polyol dispersions which are prepared directly in aqueous phase by the technique of emulsion polymerization. In this kind of technique the use of external emulsifiers which bring about electrostatic or steric stabilization is generally necessary.
Highly suitable as rapidly physically drying aqueous 2K PUR systems are, for example, those which comprise primary dispersions as polyol components. These polyol components have molar weight values of Mn significantly greater the 5000 g/mol and Mw values of generally greater than 30 000 g/mol. These primary dispersions can generally be combined only with hydrophilicized or partially hydrophilicized polyisocyanates, thereby allowing relatively uncomplicated dispersion with relatively simple stirring assemblies or by hand and glass rod (see, e.g., EP-A 0 557 844). Additionally, such aqueous 2K PUR systems customarily include relatively high concentrations of organic cosolvents, which act as coalescence aids when a film is formed on a substrate. When such aqueous, cosolvent-containing 2K PUR systems are applied as coatings, for example to mineral substrates, such as floors, for example, coatings are obtained which exhibit very good levelling, relatively high gloss levels and low haze values, but the volatile organic compounds (VOCs) have a severely adverse effect on the environment and may even be a hygiene burden.
Oligoether or polyether polyols are likewise known from the prior art as what are termed reactive diluents in aqueous vinyl polymer dispersions. For example, EP-A 0 557 844 describes how the polyvinyl polyol dispersions of the primary dispersion type which are used there in an aqueous 2K PUR system may further comprise up to 10% by weight, based on the overall weight of the polymer resins, of water-soluble polyhydric alcohols such as, for example, ethylene glycol, propylene glycol, glycerol, trimethylolpropane or the low molecular weight, water-soluble alkoxylation products of such polyhydric alcohols.
EP-A 0 758 007 discloses aqueous crosslinkable binder dispersions A) of low solvent content which comprise at least one hydroxy-functional polyether A1) and a hydroxy-functional vinyl polymer resin synthesized in two stages from a relatively hydrophobic component A2) and a relatively hydrophilic component A3). These hydroxy-functional vinyl polymers are secondary dispersions, which are prepared by free-radical polymerization in a homogeneous organic medium in the presence of low molecular weight volatile solvents and the hydroxy-functional polyethers A1) mentioned and are then dispersed in an aqueous phase together with neutralizing agents. Although it is possible in principle to remove the organic cosolvents from these binder dispersions by distillation, the additional use of cosolvents in the binder/crosslinker systems as auxiliaries for better compatibility of the components (binder and crosslinker) and also for better film formation, is regarded as being unavoidable.
It was therefore an object of the present invention to provide polyol dispersions as binders for aqueous 2K PUR systems where there is no need to add cosolvents and which can be used as coating materials for any desired substrates.